1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of isomeric dinitrotoluenes. More specifically, the invention relates to a process for separating 2,3-dinitrotoluene and 3,4-dinitrotoluene from the other dinitrotoluene isomers by employing a solid bed adsorption system.
2. Background Information
The isomers, 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) are important starting materials for making polyurethanes which are useful in many applications as rigid or flexible forms or as fibers, e.g., insulation, soundproofing, interlinings for clothing and sleeping bags, cushions, spandex, etc. Dinitrotoluene (DNT) is reduced to the diamine (TDA) and then converted to the diisocyanate (TDI) by reaction with carbonyl chloride as in Equation 1: ##STR1## The toluene diisocyanate (TDI) is then reacted with a polyol (represented here generically by ##STR2## to give a polyurethane: ##STR3##
It is common industrial practice to make polyurethane from a mixture of the isomers, 2,4- and 2,6-toluene diisocyanate (TDI), for example 80/20 or 65/35, derived from 2,4- and 2,6-dinitrotoluene. These are obtained through the above sequence in Equation 1 from, for example, an 80/20 mixture of 2,4-/2,6-dinitrotoluene isomers. These DNT mixtures are not obtained pure, however, and they will contain up to 5% of the 2,3- and 3,4- isomers, herein referred to as minor isomers. However, the presence of about 3% of the minor isomers, 2,3- and 3,4-dinitrotoluene in the starting material, leads to significant loss of TDI yield due to the formation first of the corresponding 2,3- and 3,4-toluenediamines (TDA) in the reduction step and then the corresponding substituted ureas in the carbonylation step (Equation 1). Subsequent reaction of the urea active hydrogens with TDI reduces the TDI yield and forms a high molecular weight material which can foul process equipment. ##STR4##
Moreover, the presence of the minor DNT isomers in the reduction step (Equation 1) means additional cost is incurred reducing their nitro groups, which are of no value in TDI production. Accordingly, it is desirable to remove the minor DNT isomers from crude DNT used for the production of TDI.
One conventional scheme for avoiding the loss of TDI yield in the carbonylation step is to remove 2,3- and 3,4-TDA by distillation. However, due to the high boiling points of the TDA isomers and the small differences in the boiling points of the individual isomers, this is a difficult and energy intensive method of separation. Furthermore, it does not solve the problem of unnecessary reduction of the minor DNT isomers. Another scheme which attempts to avoid this unnecessary reduction is the reaction of aqueous sulfite solution with crude DNT. This reacts with the minor isomers to effect their removal. However, some reaction with the major 2,4- and 2,6-DNT also occurs which lowers the recovery of these desirable isomers.
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. For example, normal paraffins can be separated from branched chain paraffins with a type A zeolite and olefinic hydrocarbons can be separated from paraffinic hydrocarbons with type X or type Y zeolites. Also, X and Y zeolites have been employed in processes to separate individual hydrocarbon isomers, for example, to separate alkyl-trisubstituted benzene isomers, to separate alkyl-tetrasubstituted monocyclic aromatic isomers and to separate specific alkyl-substituted naphthalenes. Perhaps the most well known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene from a mixture of C.sub.8 aromatics.
It is known from U.S. Pat. No. 3,069,470 to Fleck et al, to use type X zeolites for the separation of the meta isomer from other isomers of toluidine. From U.S. Pat. No. 4,480,129, it is known that X and Y type zeolites, exchanged with transition metals, are para-selective in a mixture of isomers of toluidine.
In Japanese Patent Application No. 56905/79, publicly disclosed on Nov. 20, 1980, it is disclosed that a solid adsorbent containing titanium oxide will selectively adsorb the para-isomer of toluidine.
It is known from U.S. Pat. No. 4,270,013 to Priegnitz et al that ortho-nitrotoluene may be separated from other nitrotoluene isomers by using a type X zeolite containing at exchangeable cationic sites one cation selected from a group that includes potassium and barium. The specific desorbent materials disclosed by this reference are toluene and 1-hexanol. The separation of isomers of disubstituted benzenes with crystalline aluminosilicates having silica/alumina mole ratio of at least 12 is disclosed in U.S. Pat. No. 4,467,126 to Zinnen.
In our copending application Ser. No. 782,674 now U.S. Pat. No. 4,642,397 filed of even date herewith, we disclosed the separation of 2,4-dinitrotoluene from admixtures with 2,6-dinitrotoluene with adsorbents selected from X- and Y-type zeolites whose cations at exchangeable sites have been exchanged with one of the following metals: K, Na, Ca, Li or Mg.